Curiously I have a feeling that diamond jewellery falls further when you just look at the short term. What I mean is that (according to a few things I've read, and neatly summarised at http://diamondssuck.com) immediately after purchase the resale value of your diamond ring hits the floor - whereas you can at least punt a GPU for a short while afterwards.

This is mostly beside the point though - you're right that GPUs are effectively obsolete after a very short period and have very little value in the long term :)